This invention relates to the snubbing, i.e. braking, of a D.C. motor and particularly to the snubbing of a motor in a points machine.
A points machine is a device used on a railway for the operation of points, crossings and similar equipment. Typically a points machine controls the movement of displaceable sections of track. The track sections are connected to rods or levers which are actuated by means of a motor, the rotary motion of the motor being translated into linear motion by means of a ball-screw or similar device. In the interests of safety it is essential that the movement effected by the points machine be positive; in other words the track section must always be placed in the correct position. However where use is made of an electric motor in the points machine, this requirement can cause problems, for although the electrical supply to the motor may be interrupted once the track sections are correctly positioned the momentum of the rotating rotor of the motor tends to continue the movement of the ball screw, and when this is in a limiting position the resulting strain on the parts of the points machine can cause serious damage.
Where a motor is a D.C. motor snubbing is generally effected by switching the motor into a regenerative mode. To the best of the applicant's knowledge this is done by means of mechanical switches and mechanical devices which detect the sense of rotation of the D.C. motor. However the operation of these switches, and other devices which rely on an external power supply, is not consistent, particularly where the points machine is installed in a hostile environment.